Let me start off with the comment that I have been living in Japan for more than 15 years and that I'm always amazed at the fact that so many people think a bunch of fancy looking California rolls are sushi. Sushi mostly comes in form of a piece of rice with fish on top. Most of the rolls you see in so called sushi restaurants in the West are unthinkable in a Japanese sushi restaurant. Also, the majority of sushi places, just like Sushi Circle, are not run by Japanese (but by Chinese, Vietnamese, etc) and it shows in the menu, variety of fish, clientele, and service. As a general rule, if there are no Japanese customers, don't bother going in.
Having said that, my wife and I gave Sushi Circle a try based on some of the reviews here and on other sites. We just wanted to check it out once. The restaurant we visited in Frankfurt was the one close to the old Opera.
Upon entering, we realized that there were no Japanese customers in sight, bad omen. After sitting down, we ordered some Japanese green tea which turned out to be something else but not the green powdery green tea you get in Japanese sushi places. Besides, the green tea is always free and all you can drink in a real sushi place.
Next we looked at what was on offer on the sushi-belt. Most sushi dishes were some 'fantasy' California roll creations you wouldn't find in Japan. We stayed way clear of them.
We decided to sample some salmon sushi (not the roll type) and some 'saba' mackerel sushi. No other real sushi seemed to be on offer. In both cases, the sushi rice tasted dry and the texture wasn't comparable to Japanese sushi rice. The piece of mackerel on top was totally dried up and tasted like dried fish, horrible. The salmon tasted just OK, but was nothing spectacular. We then had some tuna rolls wrapped in seaweed which tasted bad as well. The tuna was bland and the seaweed was not fresh at all. The wasabi had a weird light green-brown color, a cream like texture, and wasn't 'spicy' at all. it was nothing like the high-grade wasabi you find in good sushi restaurants.
All staff in sight were non Japanese Asians and definitely not trained sushi experts. They were friendly though.
For what we got, the price was definitely too much. Again, this is not a real sushi nor a real Japanese style 'kaiten' (converter belt sushi) restaurant. There's barely any real sushi on the menu. I would describe it as the McDonald's of sushi suitable for customers who never had real sushi before.
We will definitely not try a Sushi Circle restaurant again.